That last year or so, things got rough. I was really pushing the limit of what it could run without feeling unreasonably sluggish (with pretty normal workloads of browsing, IM, and either word processing or Ruby coding). First, I switched from Firefox to Opera. Then, I switched from Gnome to Xfce to IceWM. This made a world of difference, and I really didn't miss Gnome. It got me another 6 months+ out of that computer without being terribly frustrated all the time.
In most cases, on a modern computer where you have multiple gigabytes of RAM, I agree with you. But it's still nice to have those tiny window managers around.
So although I agree with your point it's nice to not have everything boil down to dollars and cents all the time :)
I'm very much a 'not dollars and cents' guy, but I see a lot of people struggle to maintain old computers when they're not really into enjoying that maintenance activity; those for whom an hour of troubleshooting -foo- is pure frustration with no fun mixed in.
Also there's a fair number of machines out there, used for non-web browsing purposes, yet might still have a use for a GUI.
Lets say you want a virtual cloud host, connected to via VNC to ... linode. The smallest linode is a gig, so you'll be using 1/5 of your total resources just for the window manager if you use KDE. Is that relevant, well only you can decide.
Finally the most interesting part of the graph was the exponential-ish nature of it. For some weird reason I was guessing WMs/Desktops grow vaguely linearily over time so a random sample from random aged WMs would be vaguely linear when graphed... that's not whats happening. Have not come up with an adequate mental model to explain the shape of the curve yet, assuming the data gathering wasn't manipulated to specifically generate it. Maybe its an attractor effect where one pole pulls to 0 or as close as you can get, and the other pole pulls to infinity, or as close as you can get (kde) and everything smoothly distributes in the middle ground.
Uselessly crude. What matters is how much code is on the critical path and how long it takes to run, not how much is loaded into RAM. A JPEG library loaded into memory that is not used when switching between windows doesn't impact window switching time. Contrariwise, KDE may have many thousands of times the latency if it tries to do many DBUS calls or something for an action that a simpler window manage would simply perform in-process. And it may not matter because "many thousands" of times the latency may still end up below the human perceptual threshold anyhow; 5ns vs 5ms is a whole lotta difference, except it sort of isn't when it comes to humans, both are instant.
Also, given the memory need of modern webapps, I wonder if even the delta between the two extremes amoutns to more than one or two opened tabs. Of course, if one or two tabs is all that your machine can handle, this is significant.
Why does every choice need a 'business case?'